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"Thermoacoustics for liquefaction of natural gas,"
G. W. Swift and J. J. Wollan,
GasTIPS, Volume 8, Number 4, pages 21-26 (Fall 2002).
(Erratum: page 23, column two, under the heading "How the refrigerator works":
The first word of the 23rd line in that paragraph should be
"falls" instead of "rises.")

Abstract:
A new type of thermoacoustic engine based on traveling waves
and ideally reversible heat transfer is described.
Measurements and analysis of its performance are presented.
This new engine
outperforms previous thermoacoustic engines, which are based on standing
waves and intrinsically irreversible heat transfer, by more than 50%.
At its most efficient
operating point, the engine delivers 710 W of acoustic power
to its resonator with a
thermal efficiency of 0.30, corresponding to 41% of the Carnot
efficiency. At its most powerful operating point, it
delivers 890 W to its resonator with a thermal efficiency of 0.22. The
efficiency of this engine can be degraded by two types of
acoustic streaming.
These are suppressed by appropriate tapering of crucial surfaces in the engine
and by using additional nonlinearity to induce an opposing time-averaged
pressure difference. Data are presented which
show the nearly complete elimination of the streaming convective heat
loads. Analysis of these and other irreversibilities show which components of
the engine require further research to attain higher efficiency.
Additionally, these data show that the dynamics and acoustic
power flows are well understood, but the details of the streaming
suppression and associated heat convection are only qualitatively
understood.

Abstract:
Condensation may occur in an open-flow thermoacoustic cooler with stack
temperatures below the saturation temperature of the flowing gas.
In the experimental device described here the flowing gas, which is
also the acoustic medium, is humid air, so the device acts as a flow-through
dehumidifier. The humid air stream flows through an acoustic resonator.
Sound energy generated by electrodynamic drivers produces a high-amplitude
standing wave inside of the resonator, which causes cooling on a
thermoacoustic stack. Condensation of water occurs as the humid air
passes through the stack and is cooled below its dew point, with the
condensate appearing on the walls of the stack. The dry, cool air
passes out of the resonator, while the condensate is wicked away from
the end of the stack.
Thermoacoustic heat pumping is strongly affected by the form of the
condensate inside of the stack, whether condensed mostly on the stack
plates, or largely in the form of droplets in the gas stream.
Two simple models of the effect of the condensate are matched to a
measured stack temperature profile; the results suggest that the
thermoacoustic effect of droplets inside the stack is small.

Abstract:
The theory for thermoacoustic boundary-layer mixture separation
is extended to include the effects of a nonzero concentration gradient.
New data are presented, which are in excellent agreement with this
theory. The maximum concentration gradient which may be achieved in a
binary mixture of gases through this separation process is
intrinsically limited by the fractional pressure amplitude, by the
tidal displacement, and by the size of the thermal diffusion ratio.
Ordinary diffusion further detracts from the attainable final
concentration gradient. Rayleigh streaming also works against
thermoacoustic separation, and an estimate of the molar flux from
streaming is given.

Abstract:
We report observation of a new mixture-separation process:
an insonified mixture of helium and argon in a narrow duct
spatially separates along the acoustic-propagation axis.
We measure mole-fraction differences across the ends of the duct as large
as 7%.
We measure initial separation flux densities as high as
0.001 times M squared times c,
where M is the acoustic Mach number and c is the sound speed.
This initial separation flux,
as a function of both the amplitudes and the relative phasing of
the pressure and velocity oscillations
in the duct, agrees well with a recent theory
involving oscillating thermal diffusion in the acoustic boundary layer.

Abstract:
Measurements of the spatial distribution of the time-averaged second-order
pressure in a plane standing wave in atmospheric air are reported. Several
measurement pitfalls are identified, and solutions are described. These
include accounting for slight nonlinearity of the piezoresistive
transducer and careful mounting of the transducer. Streaming
causes extra complication when a capillary-connected monometer is used.
With the proper technique and instrumentation, results are in good
agreement with theory.

Abstract:
The acoustic power loss in the thermoacoustic mixture-separation process is derived, including the contributions due to a nonzero gradient in concentration. The significance of the gradient-dependent term is discussed. The limiting thermodynamic efficiency of the separation is calculated. Under reasonable circumstances, the efficiency approaches 0.01 times nH times nL times ( delta m / m avg ) squared, where nH and nL are the mole fractions of the two components of the mixture, and delta m / m avg is the fractional difference between the molar masses of the two components. This efficiency is of the same order of magnitude as that of some other, more conventional separation methods.

Abstract:
Thermoacoustic-Stirling hybrid engines and feedback pulse tube refrigerators can utilize jet pumps
to suppress streaming that would otherwise cause large heat leaks and reduced efficiency. It is
desirable to use jet pumps to suppress streaming because they do not introduce moving parts such
as bellows or membranes. In most cases, this form of streaming suppression works reliably.
However, in some cases, the streaming suppression has been found to be unstable. Using a simple
model of the acoustics in the regenerators and jet pumps, a stability criterion is derived
that predicts when jet pumps can reliably suppress streaming.

Abstract:
A cascade thermoacoustic engine is described, consisting of one standing-wave engine plus two traveling-wave
engines in series. Most of the acoustic power is produced in the efficient traveling-wave stages. The
straight-line series configuration is easy to build and allows no Gedeon streaming. The engine delivers up
to 2 kW of acoustic power, with an efficiency (ratio of acoustic power to heater power) up to 20%.
Understanding of the pressure and volume-velocity waves is very good. Agreement between measured and
calculated powers and temperatures is reasonable. Some of the measured thermal power that cannot be
accounted for by calculation can be attributed to Rayleigh streaming in the two thermal buffer tubes with
the largest aspect ratios. Straightforward extension of this work should yield cascade thermoacoustic
engines with efficiencies around 35-40% of the Carnot efficiency.

Abstract:
Experiments on oscillating flow at the abrupt transition between a 2-D channel and essentially
infinite space are presented. It is shown that phenomena associated with the transition are
functions of three independent dimensionless parameters including the dimensionless radius
rounding the edge of the end of the channel. The effect of each of these
three parameters on the time-averaged pressure difference across the transition and the acoustic
power dissipation is explored by holding two parameters fixed while varying the third. Evidence is
presented that the losses due to oscillatory flow in this geometry are smaller than would be
expected from commonly accepted values for steady flow in similar geometry.

Abstract:
The enrichment of the neon isotopes in a thermoacoustic device is
demonstrated. Because the thermal diffusion ratio of neon is small, an
apparatus longer than a wavelength was necessary in order to easily observe
the separation. The device was modular and extensible, so that arbitrarily
large separations could in principle be obtained. The acoustic duct was a
series of multiple, identical quarter-wavelength modules with side-branch
drivers. In this way, waveforms close to that of a traveling wave were
maintained in the duct, despite the high acoustic attenuation caused by the
duct's small diameter and large length. The concentrations of the isotopes
were measured at one end of the duct using a quadrupole mass spectrometer.
For the operating frequency of 227 Hz, the maximum separation gradient
obtained was 0.43%/m, and mole fluxes at zero gradient as high as 3 nmol/s
were observed. Effects of turbulence, though not observed, are also
discussed, and the scaling properties of this method are compared with those
of traditional mixture-separation methods.

Abstract:
Future NASA deep-space missions will require radioisotope-powered
electric generators that are just as reliable as current-RTGs, but more
efficient and of higher specific power (W/kg). Thermoacoustic engines at the
similar to1kW scale have converted high-temperature heat into, acoustic, or
PV, power without moving parts at 30% efficiency. Consisting of only tubes
and a few heat exchangers, thernibacoustic engines are low mass and promise
to be highly reliable. Coupling a thermoacoustic engine to a low mass,
highly reliable and efficient linear alternator will create a heat-driven
electric generator suitable for deep-space applications. Conversion
efficiency data will be presented on a demonstration thermoacoustic engine
designed for the 100-Watt power range.

Abstract:
A thermoacoustic power converter for use in space in the
conversion of radioisotope-generated heat to electricity is under
development. The converter incorporates a thermoacoustic driver that
converts heat to acoustic power without any moving parts. The acoustic power
is used to drive a pair of flexure bearing supported pistons connected to
voice coils in a vibrationally balanced pair of moving coil alternators.
Initial tests of the small similar to100W thermoacoustic driver have
demonstrated good efficiency. An alternator matched to the driver is now
under construction. A description of the system and the results of
development tests are presented.

Abstract:
Future NASA deep-space missions will require radioisotope-powered
electric generators that are just as reliable as current RTGs, but more
efficient and of higher specific power (W/kg). Thermoacoustic engines can
convert high-temperature heat into acoustic, or PV, power without moving
parts at 30% efficiency. Consisting of only tubes and a few heat exchangers,
these engines are low mass and promise to be highly reliable. Coupling a
thermoacoustic engine to a low-mass, highly reliable and efficient linear
alternator will create a heat-driven electric generator suitable for
deep-space applications. Data will be presented on the first tests of a
demonstration thermoacoustic engine designed for the 100-Watt power range.

Abstract:
Traveling-wave thermoacoustic heat engines have been demonstrated to convert high-temperature heat to acoustic power with high efficiency without using moving parts. Electrodynamic linear alternators and compressors have demonstrated high acoustic-to-electric transduction efficiency as well as long maintenance-free lifetimes. By optimizing a small-scale traveling-wave thermoacoustic engine for use with an electrodynamic linear alternator, we have created a traveling-wave thermoacoustic electric generator; a power conversion system suitable for demanding applications such as electricity generation aboard spacecraft.

Abstract:
Thermoacoustic engines and refrigerators use the interaction between heat and sound to produce acoustic energy or to transport thermal energy. Heat leaks in thermal buffer tubes and pulse tubes, components in thermoacoustic devices that separate heat exchangers at different temperatures, reduce the efficiency of these systems. At high acoustic amplitudes, Rayleigh mass streaming can become the dominant means for undesirable heat leak. Gravity affects the streaming flow patterns and influences streaming-induced heat convection. A simplified analytical model is constructed that shows gravity can reduce the streaming heat leak dramatically.

Abstract:
Vortex shedding that occurs in ducts with baffles in the presence of mean flow often leads to excitation of acoustic modes. Resulting flow oscillations may feed back to the process of vortex formation. A simple model is proposed for describing this complex interaction using the hypotheses for a quasi-steadiness of vortex shedding and for a short-period acoustic perturbation at the moment of vortex collision with a downstream baffle. The model is capable of predicting typical real-system phenomena, such as the lock-in of a dominant frequency of the vortex-acoustic instability in some ranges of the mean flow velocity.

Abstract:
An asymmetrical constriction in a pipe functions as an imperfect gas diode
for acoustic oscillations in the pipe. One or more gas diodes in a loop of
pipe create substantial mean flow, approximately proportional to the
amplitude of the oscillations. Measurements of wave shape, time-averaged
pressure distribution, mass flow, and acoustic power dissipation are
presented for a two-diode loop. Analysis of the phenomena is complicated
because both the mean flow and the acoustic flow are turbulent and each
affects the other significantly. The quasi-steady approximation yields
results in rough agreement with the measurements. Acoustically driven
heat-transfer loops based on these phenomena may provide useful heat
transfer external to thermoacoustic and Stirling engines and refrigerators.

Abstract:
A major technical hurdle to the implementation of large Stirling engines or thermoacoustic engines is the reliability, performance, and manufacturability of the hot heat exchanger that brings high-temperature heat into the engine. Unlike power conversion devices that utilize steady flow, the oscillatory nature of the flow in Stirling and thermoacoustic engines restricts the length of a traditional hot heat exchanger to a peak-to-peak gas displacement, which is usually around 0.2 meters or less. To overcome this restriction, a new hot heat exchanger has been devised that uses a fluid diode in a looped pipe, which is resonantly driven by the oscillating gas pressure in the engine itself, to circulate the engine's working fluid around the loop. Instead of thousands of short, intricately interwoven passages that must be individually sealed, this new design consists of a few pipes that are typically 10 meters long. This revolutionary approach eliminates thousands of hermetic joints, pumps the engine's working fluid to and from a remote heat source without using moving parts, and does so without compromising on heat transfer surface area. Test data on a prototype loop integrated with a 1-kW thermoacoustic engine will be presented.

Abstract:
Thermoacoustic and Stirling engines and refrigerators use heat exchangers to transfer heat between
the oscillating flow of their thermodynamic working fluids and external heat sources and sinks. An
acoustically driven heat-exchange loop uses an engine's own pressure oscillations to steadily
circulate its own thermodynamic working fluid through a physically remote high-temperature heat
source without using moving parts, allowing for a significant reduction in the cost and complexity
of thermoacoustic and Stirling heat exchangers. The simplicity and flexibility of such
heat-exchanger loops will allow thermoacoustic and Stirling machines to access diverse heat sources
and sinks. Measurements of the temperatures at the interface between such a heat-exchange loop
and the hot end of a thermoacoustic-Stirling engine are presented. When the steady flow is too small
to flush out the mixing chamber in one acoustic cycle, the heat transfer to the regenerator is
excellent, with important implications for practical use.

Abstract:
The superposition of nonzero time-averaged mole flux Ndot on a thermoacoustic wave in a binary gas
mixture in a tube produces continuous mixture separation, in which one or more partially purified
product streams are created from a feedstock stream. Significant product and feedstock flows occur
through capillaries that are small enough to experience negligible thermoacoustic phenomena of
their own. Experiments with a 5050 helium-argon mixture show diverse consequences of nonzero
flow, involving the addition of only one simple term n_H to the equation for the heavy component's
time-averaged mole flux, where n_H is the mole fraction of the heavy component. A boundary
condition for n_H must be imposed on the equation wherever products flow out of the separation tube,
but not where feedstock flows in.

Abstract:
Various oscillating-wave thermodynamic devices, including orifice and feedback pulse tube refrigerators,
thermoacoustic-Stirling hybrid engines, cascaded thermoacoustic engines, and traditional Stirling engines
and refrigerators, utilize regenerators to amplify acoustic power (engines) or to pump heat acoustically
up a temperature gradient (refrigerators). As such a regenerator is scaled to higher power or operated
at lower temperatures, the thermal and hydrodynamic communication transverse to the acoustic axis
decreases, allowing for the possibility of an internal acoustic streaming instability with regions of
counterflowing streaming that carry significant heat leak down the temperature gradient. The instability is
driven by the nonlinear flow resistance of the regenerator, which results in different hydrodynamic flow
resistances encountered by the oscillating flow and the streaming flow. The instability is inhibited by
several other mechanisms, including acoustically transported enthalpy flux and axial and transverse thermal
conduction in the regenerator solid matrix. A calculation of the stability limit caused by these effects
reveals that engines are immune to a streaming instability while, under some conditions, refrigerators can
exhibit an instability. The calculation is compared to experimental data obtained with a specially built
orifice pulse tube refrigerator whose regenerator contains many thermocouples to detect a departure from
transverse temperature uniformity.

Abstract:
The distortion of temperature profiles at the ends of thermal buffer tubes is related to the time-dependent gas pressure and motion in both nearly adiabatic and nearly isothermal environments during one acoustic cycle. The analytical solution for the mean temperature distribution is derived assuming zero heat conduction between gas parcels and linear acoustics with the acoustic wavelength much longer than other system dimensions. Theoretical results are compared with some experimental data and with results of numerical simulations that assume high heat conductivity.

Abstract:
The theory of thermoacoustic mixture separation is extended to include the effect of a nonzero axial
temperature gradient. The analysis yields a new term in the second-order mole flux that is
proportional to the temperature gradient and to the square of the volumetric velocity and is
independent of the phasing of the wave. Because of this new term, thermoacoustic separation stops
at a critical temperature gradient and changes direction above that gradient. For a traveling wave,
this gradient is somewhat higher than that predicted by a simple four-step model. An experiment
tests the theory for temperature gradients from 0 to 416 K/m in 50-50 He-Ar mixtures.

Abstract:
An inverted pulse tube in which gravity-driven convection is suppressed by acoustic oscillations is
analogous to an inverted pendulum that is stabilized by high-frequency vibration of its pivot point.
Gravity acts on the gas density gradient arising from the end-to-end temperature gradient in the
pulse tube, exerting a force proportional to that density gradient, tending to cause convection when
the pulse tube is inverted. Meanwhile, a nonlinear effect exerts an opposing force proportional to the
square of any part of the density gradient that is not parallel to the oscillation direction. Experiments
show that convection is suppressed when the pulse-tube convection number
N_{ptc} = omega squared a squared sqrt( Delta T / T_{avg} ) / g / (D sin theta -L cos theta) is greater
than 1 in slender tubes, where omega is the radian
frequency of the oscillations, a is their amplitude, Delta T is the end-to-end temperature difference, T_{avg}
is the average absolute temperature, g is the acceleration of gravity, L is the length of the pulse tube
and D is its diameter, alpha is about 1.5, and the tip angle theta ranges from 90° for a horizontal tube to 180°
for an inverted tube. Theory suggests that the temperature dependence should be Delta T / T_{avg} instead of
sqrt( Delta T / T_{avg} ).

Abstract:
Thermoacoustic theory is applied to oscillating flow in a parallel-plate gap with
finite and unequal heat capacities on the two bounding walls, and with relative movement
of one wall with respect to the other. The motivation is to understand the behavior of
displacer gap losses at low temperatures in a Stirling cooler. Equations for the oscillating
temperature and enthalpy flux down the gap and down the moving solid as a function of
pressure amplitude, flow, temperatures, wall velocity, and material properties are derived.
General expressions, along with results illustrating the behavior of the solutions, are
presented. The primary result is that losses may increase significantly below 25 K, due to
vanishing wall heat capacities and reduced thermal penetration depth in the helium gas.

Abstract:
In high-power pulse-tube refrigerators, the pulse tube itself can be very
long without too much dissipation of acoustic power on its walls. The
pressure amplitude, the volume flow rate amplitude, and the time phase
between them evolve significantly along a pulse tube that is about a quarter
wavelength long. Proper choice of length and area makes the oscillations at
the ambient end of the long pulse tube optimal for driving a second, smaller
pulse-tube refrigerator, thereby utilizing the acoustic power that would
typically have been dissipated in the first pulse-tube refrigerator's
orifice. Experiments show that little heat is carried from the ambient heat
exchanger to the cold heat exchanger in such a long pulse tube, even though
the oscillations are turbulent and even when the tube is compactly coiled.

Abstract:
In a tube many wavelengths long, thermoacoustic separation of a gas mixture
can produce very high purities. A flexible wall allows a spatially
continuous supply of acoustic power into such a long tube. Coiling the tube
and immersing it in a fluid lets a single-wavelength, circulating, traveling
pressure wave in the fluid drive all the wavelengths in the tube wall and
gas. Preliminary measurements confirm many aspects of the concept with neon
(20Ne and 22Ne) and highlight some challenges of practical
implementation.